A colloid (ion electrospray) thruster includes an emitter receiving a propellant such as an ionic liquid from a reservoir and positioned near an electrode or extractor grid. A voltage is applied between the emitter and the electrode to produce a Taylor cone of ions providing thrust. See U.S. Pat. Nos. 8,785,881 and 8,324,593 incorporated herein by this reference.
To avoid the need for a propellant pressurization system onboard a spacecraft used to deliver propellant from the reservoir to the emitter, a porous propellant reservoir and a porous emitter may be used with propellant transferred from the reservoir to the emitter by capillary action. See Courtney, Daniel G. and Shea, Herbert, “Influences of Porous Reservoir Laplace Pressure on Emissions from Passively Fed Ionic Liquid Electrospray Sources,” Applied Physics Letters 107, (2015) incorporated herein by this reference.
If the reservoir is fully filled with ionic liquid, the ionic liquid can leak out of the emitter tip and form a bridge between the emitter and the electrode resulting in a failure of the thruster. See Courtney, Daniel G. et al. “Performance and Applications of Ionic Electrospray Micro-Propulsion Prototypes,” ATAA Space (2015), AIAA paper 2015-222013 incorporated herein by this reference. Partially filling the reservoir can alleviate this particular problem but then the possibility exists that they will be a void or gap of ionic liquid between the reservoir and the emitter again resulting in thruster failure.